1. Field of the Invention
The present invention relates to a communication technique in a communication system.
2. Description of the Related Art
In the WUSB 1.0 standard, a host and devices form a WUSB cluster and communicate using TDMA-based superframes (for example, see Wireless Universal Serial Bus Specification, Revision 1.0). WUSB is an abbreviation for Wireless Universal Serial Bus, and TDMA is an abbreviation for Time Division Multiple Access.
A superframe consists of 256 Media Access Slots (MASs) of 256 μs each, with the first 16 MASs being used as a BP (Beacon Period) to transmit beacons. Three beacon slots are assigned to one MAS and therefore 48 beacon slots exist in the 16 MASs, while beacon slot number 0 and number 1 cannot be used for band reservation because they are used for BP relocation.
The remaining duration of the superframe other than the BP is used as DRPs (Distributed Reservation Periods), each of which is a band in which communication is possible within a cluster. The duration of a DRP is indicated by a DRP IE (DRP Information Element) in a beacon transmitted from the host or a device.
The number of a beacon slot for transmitting a beacon is determined by negotiation among the devices. A device also uses a BPO IE (Beacon Period Occupancy IE) in the beacon in order to communicate the numbers of the beacon slots used by neighboring devices from the device to devices existing as hidden terminals.
Synchronization among the hosts and devices is managed in an autonomous-decentralized manner, and the hosts have a function of managing superframe synchronization adjustment. A host or a device performs the superframe synchronization adjustment by receiving beacons of other devices.
The devices are broadly divided into: SBDs having a function of managing the superframe synchronization adjustment by themselves; DBDs not having the function of managing the superframe synchronization adjustment by themselves; and NBDs not managing the synchronization and not transmitting or receiving beacons for further reduction in power consumption. SBD is an abbreviation for Self-Beaconing Device, DBD for Directed Beaconing Device, and NBD for Non Beaconing Device.
Now, the frame structure at the MAC (Media Access Control) layer used by the WUSB 1.0 standard will be described with reference to FIG. 1.
This standard uses the ECMA-368 standard for the MAC layer. The communication time is managed based on a frame called a superframe 300, and one superframe has a duration of 65536 μs (for example, see Standard ECMA-368 High Rate Ultra Wideband PHY and MAC Standard). The first 16 MASs in the superframe are dedicated as a BP 400, and WUSB hosts and SBDs transmit a beacon in the BP 400 to reserve a band in the superframe as a DRP. The start point of the superframe, that is, the start point of the BP 400, is called a BPST (Beacon Period Start Time). A beacon 410 consists of a beacon group parameter 4110, a DRP IE 4120, a BPO IE 4130, and other IEs (Information Elements) 4140.
A device transmitting a beacon announces in the DRP IE 4120 the position of MASs of reserved bands, and announces in the BPO IE 4130 the beacon slot used by the device, the beacon slot numbers of neighboring devices, and the BP length.
Now, the relationship between a WUSB channel and the MAC layer will be described with reference to FIG. 2. Each DRP (420, 430, 440, 450, 460, 470) in FIG. 2 is a reserved period of a communication band in a certain WUSB cluster. In each DRP, a WUSB host broadcasts MMCs (Micro-scheduled Management Commands) that control the data input/output direction and so on. An MMC consists of a header and IEs.
The section from an MMC to the next MMC is called a TG (Transaction Group); for example, the TG for an MMC 443 is a TG 444. Described in the MMC 443 are the communication band in the direction from the WUSB host to the WUSB devices, the communication band in the direction from the WUSB devices to the WUSB host, and so on.
If a WUSB host has an SBD in the WUSB cluster, the management of the superframe synchronization adjustment with a neighboring device is performed by analyzing a packet that can be received around the SBD via the SBD and by moving the position of the BPST.
During the BP period, the SBD not only transmits a beacon by itself but also receives beacons from other devices in other beacon slots and analyzes which beacon slots are available by referring to the BPO IE.
However, there has been the following problem; if the BP reaches the maximum length and the beacon slots that the BP can accommodate are occupied, the SBD cannot transmit a beacon at startup and cannot connect to the WUSB host.